This is a good list.There’s a list of postboxes online at the railway codes website here:
Post boxes at UK railway stations
UK railway stations with post boxes in themwww.railwaycodes.org.uk
I have one query though - there are a fair few on that list which say something along the lines of 'not serviced by Royal Mail', yet there's a photo attached to them of a postbox which hasn't been closed up. Does that mean staff at the station are somehow handling the mail? e.g. https://www.flickr.com/photos/maxgamble/52027390656/
I've never really given it much thought, but I'd assume anybody posting mail into what appears to be a bona fide Royal Mail postbox would expect their post to be handled solely by Royal Mail?